The Tottenham teenager impressed after coming on for a late cameo in Spurs' thumping 4-0 Europa League victory over Wolfsberger.
The 16-year-old's determination and eagerness was clear for all to see as Dele Alli inspired the Lilywhites to a comfortable win in north London.
He made a made a key intervention for the fourth and final Tottenham goal - catching Gustav Henriksson in possession to tee up Benfica loanee Carlos Vinicius' second effort of the night.
And Mourinho singled out Scarlett for special praise following his nine-minute showing in the match.
"He is a diamond, a kid with incredible potential," Mourinho told BT Sport.
"He has worked many times with the first team and that gives him a different personality. He is still 16, 17 soon [in March] and I believe next season he will be a first-team squad player because he has a lot of talent.
"He is going to be a fantastic player and I hope everything around him goes well. He is a striker, a number nine, I have been playing him from the sides similar to Marcus Rashford and he is very clever.
"There is not one player here that I say I am not happy with, the performances were good, are they all going to start on Sunday [against Burnley]? No, but we are playing Thursday, Sunday, Thursday."
It's not often a young player gets called a 'diamond' by Jose Mourinho, but Dane Scarlett has just earned that feat.